I went through all of this in 2004.
I asked PSF at Lossie if i was entitled to an AIP (we were all told on JMLC we could get it back in 2001) they said 'Yes, it's all in the book'. I applied it was checked by a SNCO in PSF and authed - I got my AIP
Fast forward 3 years - just posted into Akrotiri and bought a shiny new car, next pay slip has a cancelled deduction of over £3,000. I quick visit to PSF and I'm told i was never entitled to the AIP (only some trades were allowed it and Tech trades were not on the list and never had been). At the time there were lots of people effected by this issue.
I was told after lodged a complaint they wouldn't touch my wages while the dispute went on - they didn't but i didn't see any LOA or 'work of an unsanitary nature' money for 2 1/2 years while they clawed back the money.
I never did manage to reclaim the tax i'd paid either PSF's answer was thats not our problem and being in Cyprus made it a complete PITA to deal with the tax office.
If these guys win great, then i'm sure there are lots of guys who've been hit in the past will all be sat waiting to stick in a claim for the money they paid back.
The whole AIP system was a cluster from the start - there was no parity accross the trades, one could use the quals from JMLC, others couldn't all because the trade sponsors decided that way.
I wonder how many people have been effected by one of these 'cock ups' since the AIP system came in!